Epitaxial substrate and method for manufacturing epitaxial substrate

ABSTRACT

Provided is a crack-free epitaxial substrate having excellent breakdown voltage properties in which a silicon substrate is used as a base. The epitaxial substrate includes a (111) single crystal Si substrate and a buffer layer including a plurality of first lamination units. Each of those units includes a composition modulation layer formed of a first composition layer made of AlN and a second composition layer made of Al x Ga 1-x N being alternately laminated, and a first intermediate layer made of Al y Ga 1-y N (0≦y≦1). The relationship of x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n) and x(1)≧x(n) is satisfied, where n represents the number of laminations of each of the first and second composition layers, and x(i) represents the value of x in i-th one of the second composition layers as counted from the base substrate side. The second composition layer is coherent to the first composition layer, and the first intermediate layer is coherent to the composition modulation layer.

TECHNICAL FIELD

The present invention relates to an epitaxial substrate for use in asemiconductor device, and particularly to an epitaxial substrate made ofa group-III nitride.

BACKGROUND ART

A nitride semiconductor is attracting attention as a semiconductormaterial for a light-emitting device such as a LED or a LD and for ahigh-frequency/high-power electronic device such as a HEMT, because thenitride semiconductor has a wide band gap of direct transition type andthe breakdown electric field and the saturation electron velocitythereof are high. For example, a HEMT (high electron mobilitytransistor) device in which a barrier layer made of AlGaN and a channellayer made of GaN are laminated takes advantage of the feature thatcauses a high-concentration two-dimensional electron gas (2DEG) to occurin a lamination interface (hetero interface) due to the largepolarization effect (a spontaneous polarization effect and a piezopolarization effect) specific to a nitride material (for example, seeNon-Patent Document 1).

In some cases, a single crystal (a different kind single crystal) havinga composition different from that of a group-III nitride, such as SiC,is used as a base substrate for use in a HEMT-device epitaxialsubstrate. In this case, a buffer layer such as a strained-superlatticelayer or a low-temperature growth buffer layer is generally formed as aninitially-grown layer on the base substrate. Accordingly, aconfiguration in which a barrier layer, a channel layer, and a bufferlayer are epitaxially formed on a base substrate is the most basicconfiguration of the HEMT-device substrate including a base substratemade of a different kind single crystal. Additionally, a spacer layerhaving a thickness of about 1 nm may be sometimes provided between thebarrier layer and the channel layer, for the purpose of facilitating aspatial confinement of the two-dimensional electron gas. The spacerlayer is made of, for example, AlN. Moreover, a cap layer made of, forexample, an n-type GaN layer or a superlattice layer may be sometimesformed on the barrier layer, for the purpose of controlling the energylevel at the most superficial surface of the HEMT-device substrate andimproving contact characteristics of contact with an electrode.

The HEMT device and the HEMT-device substrate involve various problemsincluding problems concerning improvement of the performance such asincreasing the power density and efficiency, problems concerningimprovement of the functionality such as a normally-off operation,fundamental problems concerning a high reliability and cost reduction,and the like. Active efforts are made on each of the problems.

On the other hand, in the preparation of the above-mentioned nitridedevice, research and development have been made about the use of singlecrystal silicon for a base substrate for the purpose of reduction of thecost of an epitaxial substrate, furthermore, integration with asilicon-based circuit device, and the like (for example, see PatentDocuments 1 to 3, and Non-Patent Document 2). In a case where aconductive material such as silicon is selected for the base substrateof the HEMT-device epitaxial substrate, a field plate effect is appliedfrom a back surface of the base substrate, and therefore a HEMT devicecan be designed for a high breakdown voltage and high-speed switching.

It is already known that, in order that the HEMT-device epitaxialsubstrate can be structured with a high breakdown voltage, it iseffective to increase the total film thickness of the channel layer andthe barrier layer and to improve the electrical breakdown strength ofboth of the layers (for example, see Non-Patent Document 2).

A method for manufacturing a semiconductor device is also known in whichan interposed layer made of AlN is formed on a Si base substrate, then afirst semiconductor layer made of GaN and a second semiconductor layermade of AlN are alternately formed so as to cause convex warping as awhole, and then these layers are made contract at a subsequenttemperature drop, to result in cancellation of the warping of the entiresubstrate (for example, see Patent Document 4).

However, it is known that forming a nitride film of good quality on asilicon substrate is very difficult as compared with a case of using asapphire substrate or a SiC substrate, for the following reasons.

Firstly, the values of the lattice constants of silicon and nitridematerials are greatly different from each other. This causes a misfitdislocation at an interface between the silicon substrate and a growthfilm, and facilitates a three-dimensional growth mode at a timing fromthe nucleus formation to the growth. In other words, this is a factorthat hinders the formation of a good nitride epitaxial film having a lowdislocation density and a flat surface.

Additionally, the nitride material has a higher thermal expansioncoefficient value than that of silicon. Therefore, in the step oflowering the temperature to the vicinity of the room temperature after anitride film is epitaxially grown on the silicon substrate at a hightemperature, a tensile stress acts in the nitride film. As a result, itis likely that cracking occurs in a film surface and large warpingoccurs in the substrate.

Moreover, it is also known that trimethylgallium (TMG) that is amaterial gas of the nitride material for a vapor-phase growth is likelyto form a liquid-phase compound with silicon, which is a factor thathinders the epitaxial growth.

In a case where the conventional techniques disclosed in the PatentDocuments 1 to 3 and in the Non-Patent Document I are adopted, it ispossible to cause an epitaxial growth of a GaN film on the siliconsubstrate. However, the resulting GaN film never has a better crystalquality as compared with a case of using SiC or sapphire for the basesubstrate. Therefore, preparing an electronic device such as a HEMTusing the conventional techniques involves problems of a low electronmobility, a leakage current during the off-time, and a low breakdownvoltage.

Furthermore, in the method disclosed in the Patent Document 4, largeconvex warping is intentionally caused in the course of the devicepreparation. This may cause cracking in the course of the devicepreparation, depending on conditions under which the layers are formed.

PRIOR-ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Laid-Open No. 10-163528(1998)

Patent Document 2: Japanese Patent Application Laid-Open No. 2004-349387

Patent Document 3: Japanese Patent Application Laid-Open No. 2005-350321

Patent Document 4: Japanese Patent Application Laid-Open No. 2009-289956

Non-Patent Documents

Non-Patent Document 1: “Highly Reliable 250 W GaN High Electron MobilityTransistor Power Amplifier”, Toshihide Kikkawa, Jpn. J. Appl. Phys. 44,(2005), 4896.

Non-Patent Document 2: “High power AlGaN/GaN HFET with a high breakdownvoltage of over 1.8 kV on 4 inch Si substrates and the suppression ofcurrent collapse”, Nariaki Ikeda, Syuusuke Kaya, Jiang Li, YoshihiroSato, Sadahiro Kato, Seikoh Yoshida, Proceedings of the 20thInternational Symposium on Power Semiconductor Devices & IC's May 18-22,2008 Orlando, Fla.”, pp. 287-290

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems describedabove, and an object of the present invention is to provide an epitaxialsubstrate having excellent breakdown voltage properties in which asilicon substrate is used as a base substrate.

To solve the problems described above, a first aspect of the presentinvention is an epitaxial substrate in which a group of group-IIInitride layers are formed on a base substrate made of (111)-orientedsingle crystal silicon such that a (0001) crystal plane of the group ofgroup-III nitride layers is substantially in parallel with a substratesurface of the base substrate. The epitaxial substrate includes: abuffer layer including a plurality of first lamination units eachincluding a composition modulation layer and a first intermediate layer,the composition modulation layer being formed of a first compositionlayer made of AlN and a second composition layer made of a group-IIInitride having a composition of Al_(x)Ga_(1-x)N (0≦x≦1) beingalternately laminated, the first intermediate layer being made of agroup-III nitride having a composition of Al_(y)Ga_(1-y)N (0≦y≦1); and acrystal layer formed on the buffer layer. Each of the compositionmodulation layers is formed so as to satisfy the relationship of:x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n) and x(1)>x(n), where n represents thenumber of laminations of each of the first composition layer and thesecond composition layer (n is a natural number equal to or greater thantwo), and x(i) represents the value of x in i-th one of the secondcomposition layers as counted from the base substrate side. In each ofthe composition modulation layers, each of the second composition layersis formed so as to be in a coherent state relative to the firstcomposition layer. The first intermediate layer is formed so as to be ina coherent state relative to the composition modulation layer.

In a second aspect of the present invention, in the epitaxial substrateaccording to the first aspect, the buffer layer is formed of the firstlamination unit and a second lamination unit being alternatelylaminated; and the second lamination unit is a substantially strain-freeintermediate layer made of AlN and formed with a thickness of 10 nm ormore and 150 nm or less.

In a third aspect of the present invention, in the epitaxial substrateaccording to the first or second aspect, the first intermediate layer ismade of a group-III nitride having a composition of Al_(y)Ga_(1-y)N(0.25≦y≦0.4).

In a fourth aspect of the present invention, in the epitaxial substrateaccording to the first or second aspect, a termination layer having thesame composition as that of the first composition layer is provided inan uppermost portion of the composition modulation layer.

In a fifth aspect of the present invention, in the epitaxial substrateaccording to the fourth aspect, the thickness of the termination layeris greater than the thickness of the first composition layer.

In a sixth aspect of the present invention, the epitaxial substrateaccording to any of the first to fifth aspects further includes: a firstbase layer made of AlN and formed on the base substrate; and a secondbase layer made of Al_(p)Ga_(1-p)N (0≦p≦1) and formed on the first baselayer. The first base layer is a layer with many crystal defectsconfigured of at least one kind from a columnar or granular crystal ordomain. An interface between the first base layer and the second baselayer defines a three-dimensional concavo-convex surface. The bufferlayer is formed immediately on the second base layer.

A seventh aspect of the present invention is a method for manufacturingan epitaxial substrate for use in a semiconductor device, the epitaxialsubstrate having a group of group-III nitride layers formed on a basesubstrate made of (111)-oriented single crystal silicon such that a(0001) crystal plane of the group of group-III nitride layers issubstantially in parallel with a substrate surface of the basesubstrate. The method includes: a buffer layer formation step forforming a buffer layer including a plurality of first lamination unitseach including a composition modulation layer and a first intermediatelayer by performing a composition modulation layer formation step and afirst intermediate layer formation step a plurality of times, thecomposition modulation layer formation step being a step for forming thecomposition modulation layer by alternately laminating a firstcomposition layer made of AlN and a second composition layer made of agroup-III nitride having a composition of AlxGa1-xN (0≦x>1), the firstintermediate layer formation step being a step for forming the firstintermediate layer made of a group-III nitride having a composition ofAl_(y)Ga_(1-y)N (0≦y'1); and a crystal layer formation step for forminga crystal layer above the buffer layer, the crystal layer being made ofgroup-III nitride. In the composition modulation layer formation step,the composition modulation Layer is formed in such a manner that: therelationship of x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n) and x(1)>x(n) issatisfied, where n represents the number of laminations of each of thefirst composition layer and the second composition layer (n is a naturalnumber equal to or greater than two), and x(i) represents the value of xin i-th one of the second composition layers as counted from the basesubstrate side; and each of the second composition layers is in acoherent state relative to the first composition layer. In the firstintermediate layer formation step, the first intermediate layer isformed so as to be in a coherent state relative to the compositionmodulation layer.

In an eighth aspect of the present invention, in the method formanufacturing the epitaxial substrate according to the seventh aspect,the buffer layer formation step is a step for forming the buffer layerin which the first lamination unit and a second lamination unit arealternately laminated, by alternately performing a first lamination unitformation step for forming the first lamination unit and a secondlamination unit formation step for forming second lamination unit; andin the second lamination unit formation step, an intermediate layer madeof AlN with a thickness of 10 nm or more and 150 nm or less is formed asthe second lamination unit.

In a ninth aspect of the present invention, in the method formanufacturing the epitaxial substrate according to the seventh or eighthaspect, in the first intermediate layer formation step, the firstintermediate layer is formed of a group-III nitride having a compositionof Al_(y)Ga_(1-y)(0.25≦y≦0.4).

In a tenth aspect of the present invention, in the method formanufacturing the epitaxial substrate according to the seventh or eighthaspect, the first lamination unit formation step includes a terminationlayer formation step for forming a termination layer having the samecomposition as that of the first composition layer in an uppermostportion of the composition modulation layer, and the first intermediatelayer is formed on the termination layer.

In an eleventh aspect of the present invention, in the method formanufacturing the epitaxial substrate according to the tenth aspect, inthe termination layer formation step, the termination layer is formedthicker than the first composition layer.

In a twelfth aspect of the present invention, the method formanufacturing the epitaxial substrate according to any of the seventh toeleventh aspects further includes: a first base layer formation step forforming a first base layer on the base substrate, the first base layerbeing made of AlN; and a second base layer formation step for forming asecond base layer on the first base layer, the second base layer beingmade of Al_(p)Ga_(1-p)N (0≦p≦1). In the first base layer formation step,the first base layer is formed as a layer with many crystal defectsconfigured of at least one kind from a columnar or granular crystal ordomain, such that a surface thereof is a three-dimensionalconcavo-convex surface. In the buffer layer formation step, the bufferlayer is formed immediately on the second base layer.

In the first to twelfth aspects of the present invention, an epitaxialsubstrate having a high breakdown voltage is achieved while a siliconsubstrate, which is easily available in a large diameter at a low cost,is adopted as a base substrate thereof.

Particularly, in the second and eighth aspects, providing the secondlamination unit serving as a second intermediate layer causes a largecompressive strain to exist in the buffer layer. Accordingly, a tensilestress caused by a difference in a thermal expansion coefficient betweensilicon and a group-III nitride is cancelled by the compressive strain.Therefore, a crack-free epitaxial substrate having a small amount ofwarping and an excellent crystal quality can be obtained even when asilicon substrate is used as the base substrate.

Particularly, in the fifth and eleventh aspects, an epitaxial substratein which the warping is further reduced is achieved.

Particularly, in the sixth and twelfth aspects, the buffer layer isprovided on the base layer having a low dislocation and an excellentsurface flatness. Accordingly, the buffer layer, the crystal layer, andthe like, have good crystal quality. On the other hand, an accumulationof strain energy in the second base layer is suppressed. Therefore, theeffect of canceling the tensile stress exerted by the compressive strainexisting in the buffer layer is not hindered by any accumulation ofstrain energy in the base layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an outlineconfiguration of an epitaxial substrate 10 according to an embodiment ofthe present invention.

FIGS. 2A, 2B, and 2C are model diagrams showing a crystal lattice at atime when a second composition layer 32 is formed on a first compositionlayer 31 in a composition modulation layer 3.

FIG. 3 is a diagram illustrating how the Al mole fraction was changed inmain specimens according to an example.

EMBODIMENT FOR CARRYING OUT THE INVENTION

<Outline Configuration of Epitaxial Substrate>

FIG. 1 is a schematic cross-sectional view showing an outlineconfiguration of an epitaxial substrate 10 according to an embodiment ofthe present invention.

The epitaxial substrate 10 mainly includes a base substrate 1, a baselayer 2, a buffer layer 8, and a function layer 9. The buffer layer 8includes a plurality of composition modulation layers 3, a plurality oftermination layers 4, a plurality of first intermediate layers 5, and aplurality of second intermediate layers 7. In the following, the layersformed on the base substrate 1 will be sometimes collectively referredto as an epitaxial film. Here for convenience of the description, theproportion of existence of Al in the group-III elements will besometimes referred to as Al mole fraction.

The base substrate 1 is a wafer of (111) plane single crystal siliconhaving the p-type conductivity. The thickness of the base substrate 1 isnot particularly limited, but for convenience of handling, it ispreferable to use the base substrate 1 having a thickness of severalhundred μm to several mm.

Each of the base layer 2, the composition modulation layer 3, thetermination layer 4, the first intermediate layer 5, the secondintermediate layer 7, and the function layer 9 is a layer formed of awurtzite-type group-III nitride by using an epitaxial growth method suchthat a its (0001) crystal plane can be substantially in parallel with asubstrate surface of the base substrate 1. In a preferred example, theselayers are formed by a metalorganic chemical vapor deposition method(MOCVD method).

The base layer 2 is a layer provided for the purpose of enabling each ofthe above-mentioned layers to be formed thereon with a good crystalquality. To be specific, the base layer 2 is formed in such a mannerthat its dislocation density is suitably reduced and it has a goodcrystal quality at least near its surface (near an interface with thecomposition modulation layer 3). As a result, a good crystal quality isobtained in the composition modulation layer 3, and additionally in thelayers formed thereon.

In this embodiment, to satisfy the purpose, the base layer 2 is composedof a first base layer 2 a and a second base layer 2 b, as describedbelow.

The first base layer 2 a is a layer made of AlN. The first base layer 2a is a layer configured of a large number of small columnar crystals orthe like (at least one kind from columnar crystals, granular crystals,columnar domains, and granular domains) that have been grown in adirection (film formation direction) substantially perpendicular to thesubstrate surface of the base substrate 1. In other words, the firstbase layer 2 a is a layer with many defects having inferior crystalproperties, in which, although uniaxial orientation is achieved along alamination direction of the epitaxial substrate 10, many crystal grainboundaries or dislocations exist along the lamination direction. In thisembodiment, for convenience of the description, the crystal grainboundary is sometimes used as the term inclusive of domain grainboundaries and dislocations, too. In the first base layer 2 a, theinterval of the crystal grain boundaries is at most about several tensnm.

The first base layer 2 a having this configuration is formed such thatthe half width of a (0002) X-ray rocking curve can be 0.5 degrees ormore and 1.1 degrees or less and such that the half width of a (10-10)X-ray rocking curve can be 0.8 degrees or more and 1.1 degrees or less.The half width of the (0002) X-ray rocking curve serves as an index ofthe magnitude of mosaicity of a c-axis tilt component or the frequencyof screw dislocations. The half width of the (10-10) X-ray rocking curveserves as an index of the magnitude of mosaicity of a crystal rotationcomponent whose rotation axis is c-axis or the frequency of edgedislocations.

On the other hand, the second base layer 2 b is a layer formed on thefirst base layer 2 a and made of a group-III nitride having acomposition of Al_(p)Ga_(1-p)N (0≦p≦1).

An interface 11 (a surface of the first base layer 2 a) between thefirst base layer 2 a and the second base layer 2 b is athree-dimensional concavo-convex surface that reflects the outer shapesof the columnar crystals and the like included in the first base layer 2a. The fact that the interface 11 has such a shape is clearly confirmedin, for example, a HAADF (high-angle annular dark-field) image of theepitaxial substrate 10. The HAADF image is obtained by a scanningtransmission electron microscope (STEM), and is a mapping image of theintegrated intensity of electron that is inelastically scattered at ahigh angle. In the HAADF image, the image intensity is proportional tothe square of an atomic number, and a portion where an atom having agreater atomic number exists is observed more brightly (more white).Therefore, the second base layer 2 b containing Ga is observedrelatively bright, and the first base layer 2 a not containing Ga isobserved relatively dark. Thereby, the fact that the interface 11therebetween is configured as a three-dimensional concavo-convex surfaceis easily recognized.

In the schematic cross-section of FIG. 1, convex portions 2 c of thefirst base layer 2 a are located at substantially regular intervals.This is merely for convenience of illustration. Actually, the convexportions 2 c are not necessarily located at regular intervals.Preferably, the first base layer 2 a is formed such that the density ofthe convex portions 2 c can be 5×10⁹/cm² or more and 5×10¹⁰/cm² or lessand the average interval of the convex portions 2 c can be 45 nm or moreand 140 nm or less. When these ranges are satisfied, the function layer9 having, particularly, an excellent crystal quality can be formed. Inthis embodiment, the convex portion 2 c of the first base layer 2 aalways denotes a position substantially at the apex of an upward convexportion of the surface (interface I1). From the results of experimentsand observations made by the inventors of the present invention, it hasbeen confirmed that a side wall of the convex portion 2 c is formed by a(10-11) plane or (10-12) plane of AlN.

In order that the convex portions 2 c that satisfy the above-mentioneddensity and average interval can be formed on the surface of the firstbase layer 2 a, it is preferable to form the first base layer 2 a withan average film thickness of 40 nm or more and 200 inn or less. In acase where the average film thickness is less than 40 nm, it isdifficult to achieve a state where the substrate surface is thoroughlycovered with AlN while forming the convex portions 2 c as describedabove. On the other hand, when the average film thickness exceeds 200nm, flattening of an AlN surface starts to progress, to make itdifficult to form the convex portions 2 c described above.

The formation of the first base layer 2 a is performed underpredetermined epitaxial growth conditions. Here, forming the first baselayer 2 a with AlN is preferable in terms of not containing Ga whichforms a liquid-phase compound with silicon and in terms of easilyconfiguring the interface 11 as a three-dimensional concavo-convexsurface because a horizontal growth is relatively unlikely to progress.

In the epitaxial substrate 10, the first base layer 2 a that is a layerwith many defects in which the crystal grain boundaries exist isinterposed between the base substrate 1 and the second base layer 2 b inthe above-described manner. This relieves a lattice misfit between thebase substrate 1 and the second base layer 2 b, and thus an accumulationof strain energy caused by this lattice misfit is suppressed. Theabove-described ranges of the half widths of the (0002) and (10-10)X-ray rocking curves with respect to the first base layer 2 a are set asranges that can suitably suppress the accumulation of strain energy dueto the crystal grain boundaries.

However, the interposition of the first base layer 2 a causes anenormous number of dislocations originating from the crystal grainboundaries such as the columnar crystals of the first base layer 2 a topropagate in the second base layer 2 b. In this embodiment, as describedabove, the interface I1 between the first base layer 2 a and the secondbase layer 2 b is configured as a three-dimensional concavo-convexsurface, and thereby such dislocations are effectively reduced.

Since the interface I1 between the first base layer 2 a and the secondbase layer 2 b is configured as a three-dimensional concavo-convexsurface, most of the dislocations caused in the first base layer 2 a arebent at the interface I1 during the propagation (penetration) from thefirst base layer 2 a to the second base layer 2 b, and coalesce anddisappear within the second base layer 2 b. As a result, only a smallpart of the dislocations originating from the first base layer 2 apenetrates through the second base layer 2 b.

Preferably, although the second base layer 2 b is formed along the shapeof the surface of the first base layer 2 a (the shape of the interfaceI1) in an initial stage of the growth, the surface thereof is graduallyflattened along with the progress of the growth, and finally obtains asurface roughness of 10 nm or less. In this embodiment, the surfaceroughness is expressed as an average roughness ra in a region of 5 μm×5μm which has been measured by an AFM (atomic force microscope). Here, interms of obtaining a good surface flatness of the second base layer 2 b,it is preferable that the second base layer 2 b is formed of a group-IIInitride having a composition that contains at least Ga, which allows ahorizontal growth to progress relatively easily.

It is preferable that the second base layer 2 b has an average thicknessof 40 nm or more. This is because, when the average thickness is lessthan 40 nm, such problems arise that concaves and convexes caused by thefirst base layer 2 a cannot sufficiently be flattened, and that thedisappearance of dislocations having propagated to the second base layer2 b and coalesced with each other does not sufficiently occur. In a casewhere the average thickness is 40 nm or more, the reduction of thedislocation density and the flattening of the surface are effectivelyachieved. Therefore, in a technical sense, no particular limitation isput on an upper limit of the thickness of the second base layer 2 b, butfrom the viewpoint of the productivity, it is preferable that thethickness is about several μm or less.

As described above, the surface of the second base layer 2 b has a lowdislocation and an excellent flatness, and therefore the layers formedthereon have a good crystal quality.

The buffer layer 8 includes at least a plurality of unit structures 6each formed of the composition modulation layer 3, the termination layer4, and the first intermediate layer 5 being laminated in the mentionedorder. Preferably, as shown in FIG. 1, the buffer layer 8 is configuredsuch that the second intermediate layer 7 is interposed between the unitstructures 6. In this case, the second intermediate layer 7 is providedas a boundary layer between the respective unit structures 6.Alternatively, in still other words, the buffer layer 8 has aconfiguration in which the uppermost and lowermost portions thereof arethe unit structures 6, and the unit structure 6 serving as a firstlamination unit and the second intermediate layer 7 serving as a secondlamination unit are alternately and repeatedly laminated. Although FIG.1 illustrates a case where there are four unit structures 6 (6 a, 6 b, 6c, 6 d) and three second intermediate layers 7 (7 a, 7 b, 7 c), thenumbers of the unit structures 6 and the second intermediate layers 7are not limited thereto and it suffices that the number of laminationsof the composition modulation layer 3 is about 3 to 6.

The composition modulation layer 3 is a part formed by a firstcomposition layer 31 made of AlN and a second composition layer 32 madeof a group-III nitride having a composition of Al_(x)Ga_(1-x)N (0≦x≦1)being alternately laminated. In this embodiment, the i-th firstcomposition layer 31 as counted from the base substrate 1 side isexpressed as “31<i>”, and the i-th second composition layer 32 ascounted from the base substrate 1 side is expressed as “32<i>”.

The second composition layer 32 is formed so as to satisfy the followingexpressions 1 and 2, where n (n is a natural number equal to or greaterthan two) represents the number of the first composition layers 31 andthe number of the second composition layers 32, and x(i) represents theAl mole fraction x in the second composition layer 32 with respect tothe i-th second composition layer 32<i> as counted from the basesubstrate 1 side.

x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n)   (Expression 1)

x(1)>x(n)   (Expression 2)

That is, the composition modulation layer 3 has a configuration in whichthe Al mole fraction is lower in the second composition layer 32<n> thanin the second composition layer 32<1> and, at least partially, the Almole fraction x in the second composition layer 32 gradually decreasesas the second composition layer 32 is more distant from the basesubstrate 1. It is more preferable to satisfy the relationship ofx(1)≧0.8 and x(n)≦0.2.

The expressions 1 and 2 are satisfied typically by forming thecomposition modulation layer 3 in such a manner that the secondcomposition layer 32 more distant from the base substrate 1 has a lowerAl mole fraction (that is, being richer in Ga). Therefore, hereinafter,in this embodiment, it is assumed that the second composition layer 32more distant from the base substrate 1 has a lower Al mole fraction,including a case where there exist a second composition layer 32<i−1>and a second composition layer 32<i> having the same Al mole fraction x.Here, forming the second composition layer 32 in such a manner is alsoexpressed as giving a compositional grading to the second compositionlayer 32.

Since the first composition layer 31 is made of AlN and the secondcomposition layer 32 is made of a group-III nitride having a compositionof Al_(x)Ga_(1-x)N, the first composition layer 31 and the secondcomposition layer 32 are formed so as to satisfy such a relationshipthat an in-plane lattice constant (lattice length) under a strain-freestate (bulk state) is greater in the group-III nitride (Al_(x)Ga_(1-x)N)of the latter than in the group-III nitride (AlN) of the former.

Additionally, in the composition modulation layer 3, the secondcomposition layer 32 is formed so as to be coherent to the firstcomposition layer 31.

It is preferable that each first composition layer 31 is formed with athickness of about 3 nm to 20 nm, and typically 5 nm to 10 nm. On theother hand, it is preferable that the second composition layer 32 isformed with a thickness of about 10 nm to 25 nm, and typically 15 nm to35 nm. The value of n is about 10 to 40.

The termination layer 4 is a layer made of a group-III nitride havingthe same composition as that of the first composition layer 31 andformed on an uppermost portion (terminating portion) of the compositionmodulation layer 3. The termination layer 4 is formed with a thicknessequal to or greater than the thickness of the first composition layer31. Additionally, the termination layer 4 is formed with a thickness ina range that allows the first intermediate layer 5 to be formed thereonin a coherent state. To be specific, it is preferable that thetermination layer 4 is formed with a thickness of about 3 to 100 nm.Typically, the termination layer 4 is formed with a thickness of 5 to 70nm.

More preferably, the termination layer 4 is formed with a thicknessgreater than the thickness of the first composition layer 31. To bespecific, the termination layer 4 is formed with a thickness of 30 to 70nm.

The first intermediate layer 5 is a layer made of a group-III nitridehaving a composition of Al_(y)Ga_(1-y)N (0≦y≦1). The first intermediatelayer 5 is made of a group-III nitride whose in-plane lattice constantunder a strain-free state is greater than that of the group-Ill nitrideof the first composition layer 31 and the termination layer 4.Preferably, the first intermediate layer 5 is made of a group-IIInitride having a composition of Al_(y)Ga_(1-y)N (0.25≦y≦0.4). The firstintermediate layer 5 is formed so as to be coherent to the terminationlayer 4. It is preferable that the first intermediate layer 5 has athickness of roughly 100 nm or more and 500 nm or less. It is morepreferable that the first intermediate layer 5 is formed with athickness of 100 nm or more and 300 nm or less.

The second intermediate layer 7 is a layer made of a group-III nitridewhose in-plane lattice constant under a strain-free state is smallerthan that of the group-III nitride of the first intermediate layer 5.Preferably, the second intermediate layer 7 is made of the samecomposition (that is, AlN) as that of the first composition layer 31 ofthe composition modulation layer 3. The second intermediate layer 7 isnot an essential component part for the achievement of an increasedbreakdown voltage of the epitaxial substrate 10, as will be describedlater. It is preferable that the second intermediate layer 7 is formedwith a thickness of roughly 10 nm or more and 150 nm or less.

The function layer 9 is at least one layer made of a group-III nitrideand formed on the buffer layer 8. The function layer 9 is a layer thatdevelops a predetermined function in a situation where predeterminedsemiconductor layers, electrodes, and the like, are additionallyprovided on the epitaxial substrate 10 to thereby form a semiconductordevice. Accordingly, the function layer 9 is constituted of one or morelayers having a composition and a thickness appropriate for thisfunction. Although FIG. 1 illustrates a case where the function layer 9is constituted of a single layer, the configuration of the functionlayer 9 is not limited thereto.

For example, a channel layer made of high-resistivity GaN and having athickness of several μm and a barrier layer made of AlGaN, InAlN, or thelike and having a thickness of several tens nm are laminated to serve asthe function layer 9, and thereby the epitaxial substrate 10 for a HEMTdevice is obtained. That is, a HEMT device is obtained by forming a gateelectrode, a source electrode, and a drain electrode on the barrierlayer, though not shown. For forming these electrodes, a known techniquesuch as a photolithography process is applicable. In such a case, aspacer layer made of AlN and having a thickness of about 1 nm may beprovided between the channel layer and the barrier layer.

Alternatively, a concentric Schottky diode is achieved by forming onegroup-III nitride layer (for example, a GaN layer) as the function layer9 and forming an anode and a cathode thereon, though not shown. Forforming these electrodes, the known technique such as thephotolithography process is also applicable.

<Method for Manufacturing Epitaxial Substrate>

Next, a method for manufacturing the epitaxial substrate 10 will begenerally described while a case of using the MOCVD method is taken asan example.

Firstly, a (111) plane single crystal silicon wafer is prepared as thebase substrate 1. A natural oxide film is removed by dilute hydrofluoricacid cleaning. Then, SPM cleaning is performed to create a state wherean oxide film having a thickness of about several Å is formed on a wafersurface. This is set within a reactor of a MOCVD apparatus.

Then, each layer is formed under predetermined heating conditions and apredetermined gas atmosphere. Firstly, for the first base layer 2 a madeof AlN, a substrate temperature is maintained at a predetermined initiallayer formation temperature of 800° C. or higher and 1200° C. or lower,and the pressure in the reactor is set to be about 0.1 to 30 kPa. Inthis state, a TMA (trimethylaluminum) bubbling gas that is an aluminumraw material and a NH₃ gas are introduced into the reactor with anappropriate molar flow ratio. A film formation speed is set to be 20nm/min or higher, and a target film thickness is set to be 200 nm orless. Thereby, the formation of the first base layer 2 a is achieved.

For the formation of the second base layer 2 b, after the formation ofthe first base layer 2 a, a substrate temperature is maintained at apredetermined second base layer formation temperature of 800° C. orhigher and 1200° C. or lower, and the pressure in the reactor is set tobe 0.1 to 100 kPa. In this state, a TMG (trimethylgallium) bubbling gasthat is a gallium raw material, a TMA bubbling gas, and a NH₃ gas areintroduced into the reactor with a predetermined flow ratio that isappropriate for a composition of the second base layer 2 b to beprepared. Thus, NH₃ is reacted with TMA and TMG. Thereby, the formationof the second base layer 2 b is achieved.

For the formation of the respective layers included in the buffer layer8, that is, for the formation of the first composition layer 31 and thesecond composition layer 32 included in the composition modulation layer3, the termination layer 4, the first intermediate layer 5, and thesecond intermediate layer 7, subsequent to the formation of the secondbase layer 2 b, a substrate temperature is maintained at a predeterminedformation temperature of 800° C. or higher and 1200° C. or lower that isappropriate for each of the layers, and the pressure in the reactor ismaintained at a predetermined value of 0.1 to 100 kPa that isappropriate for each of the layers. In this state, a NH₃ gas and agroup-III nitride material gas (TMA and TMG bubbling gases) areintroduced into the reactor with a flow ratio that is appropriate for acomposition to be achieved in each of the layers. Thereby, the formationof the respective layers is achieved. At this time, by changing the flowratio at a timing appropriate for a set film thickness, the respectivelayers are formed in a continuous manner and with desired filmthicknesses.

For the formation of the function layer 9, after the formation of thebuffer layer 8, a substrate temperature is maintained at a predeterminedfunction layer formation temperature of 800° C. or higher and 1200° C.or lower, and the pressure in the reactor is set to be 0.1 to 100 kPa.In this state, at least one of a TMI bubbling gas, a TMA bubbling gas,and a TMG bubbling gas, and a NH₃ gas are introduced into the reactorwith a flow ratio that is appropriate for a composition of the functionlayer 9 to be prepared. Thus, NH₃ is reacted with at least one of TMI,TMA, and TMG. Thereby, the formation of the function layer 9 isachieved.

After the function layer 9 is formed, in the reactor, the temperature ofthe epitaxial substrate 10 is lowered to an ordinary temperature. Then,the epitaxial substrate 10 is taken out from the reactor and subjectedto an appropriate subsequent process (such as patterning of an electrodelayer).

<Functions and Effects of Buffer Layer>

Generally, as is the case for this embodiment as well, in a case ofpreparing an epitaxial substrate by causing a crystal layer made of agroup-III nitride to epitaxially grow on a single crystal silicon waferat a predetermined formation temperature, a tensile stress in anin-plane direction occurs in the crystal layer in the course of loweringthe temperature to the ordinary temperature after the crystal growth,because the group-III nitride has a thermal expansion coefficientgreater than that of silicon (for example, silicon: 3.4×10⁻⁶/K, GaN:5.5×10⁻⁶/K). This tensile stress is a factor that causes occurrence ofcracking and warping in the epitaxial substrate. In this embodiment, thebuffer layer 8 is provided in the epitaxial substrate 10 for the purposeof reducing the tensile stress and suppressing occurrence of crackingand warping. More specifically, due to functions and effects exerted byeach of the layers included in the buffer layer 8, occurrence ofcracking and warping in the epitaxial substrate 10 are suppressed. Inthe following, a detailed description will be given.

(Composition Modulation Layer)

FIG. 2 is a model diagram showing a crystal lattice at a time when thesecond composition layer 32 is formed on the first composition layer 31in the composition modulation layer 3. Here, the lattice length, in thein-plane direction, of Al_(x)Ga_(1-x)N of the second composition layer32 under the strain-free state is defined as α₀, and the actual latticelength thereof is defined as α. In this embodiment, as shown in FIGS. 2Aand 2B, a crystal growth progresses in the second composition layer 32while keeping aligned with the crystal lattice of the first compositionlayer 31. This means that a compressive strain of s=α₀−α occurs in thein-plane direction of the second composition layer 32 during the crystalgrowth. That is, the crystal growth of the second composition layer 32progresses with strain energy held therein.

As the growth advances, energy instability increases. Therefore, amisfit dislocation is gradually introduced in the second compositionlayer 32, for releasing the strain energy. Then, upon reaching a certaincritical state, the strain energy held in the second composition layer32 is fully released. At this time, a state of α=α₀ is created as shownin FIG. 2C.

However, if the formation of the second composition layer 32 isterminated in a state of α₀>α as shown in FIG. 2B prior to reaching thestate shown in FIG. 2C, the second composition layer 32 remains holdingthe strain energy (remains containing the compressive strain). In thisembodiment, such a crystal growth with the strain energy containedtherein is referred to as a crystal growth in a coherent state. In otherwords, the second composition layer 32 is in the coherent state relativeto the first composition layer 31 as long as the second compositionlayer 32 is formed with a thickness smaller than a critical filmthickness at which the strain energy is fully released. Alternatively,in still other words, the second composition layer 32 is in the coherentstate relative to the first composition layer 31 as long as the latticelength α of the uppermost surface of the second composition layer 32(the surface that will be in contact with the first composition layer 31located immediately above) satisfies α₀>α. Even if α₀=α is createdpartially in the second composition layer 32, it can be said that thesecond composition layer 32 is in the coherent state relative to thefirst composition layer 31, as long as the second composition layer 32contains the strain energy in the above-described manner.

The in-plane lattice constant of AlN of the first composition layer 31is smaller than the in-plane lattice constant of Al_(x)Ga_(1-x)N of thesecond composition layer 32. Therefore, even when the first compositionlayer 31 is formed on the second composition layer 32 with the strainenergy held therein, the coherent state is maintained, not causing arelease of the strain energy held in the second composition layer 32located immediately below. Then, if the second composition layer 32 isagain grown on this first composition layer 31 so as to make thecoherent state, the same compressive strain as described above is alsocaused in this second composition layer 32, too.

Subsequently, in the same manner, the formation of the first compositionlayer 31 and the second composition layer 32 is alternately repeatedwhile maintaining the growth in the coherent state. Thereby, the strainenergy is held in each of the second composition layers 32. Moreover, inthis embodiment, the composition modulation layer 3 is formed such thatthe (Expression 1) and (Expression 2) are satisfied, in other words,such that the second composition layer 32<i> more distant from the basesubstrate 1 has a lower Al mole fraction x(i). Therefore, the differencebetween the in-plane lattice constant of Al_(x)Ga_(1-x)N of the secondcomposition layer 32 and the in-plane lattice constant of AlN of thefirst composition layers 31 that interpose the second composition layer32 therebetween increases in a portion more distant from the basesubstrate 1. As a result, as the second composition layer 32 is formedupper, a larger compressive strain is contained therein. Accordingly,the composition modulation layer 3 can be considered as astrain-introduced layer configured such that a portion thereof locatedmore distant from the base substrate 1 has a larger compressive straincontained therein. Such an introduction of the compressive strain intothe composition modulation layer 3 is suitably achieved in a case wherethe relationship of x(1)≧0.8 and x(n)≦0.2 is satisfied.

This compressive strain acts in a direction exactly opposite to thedirection of the tensile stress that is caused by a difference in thethermal expansion coefficient, and therefore functions to cancel thetensile stress at the time of temperature drop. In outline, the tensilestress is cancelled by a force that is proportional to the total sum ofthe magnitudes of the compressive strains contained in the n secondcomposition layers 32.

The first composition layer 31 is interposed between the two secondcomposition layers 32. The first composition layer 31 having too small athickness is not preferable, because this reduces the compressive strainoccurring in the second composition layer 32, and rather, the tensilestress is likely to exist in the first composition layer 31 itself. Onthe other hand, too large a thickness is not preferable, either, becausethe second composition layer 32 itself is likely to receive a force in atensile direction. The above-mentioned requirement that the thickness isabout 3 to 20 nm is preferable in terms of not causing such failures.

(Termination Layer)

The termination layer 4 is formed in the uppermost portion of thecomposition modulation layer 3, and made of a group-III nitride havingthe same composition as that of the first composition layer 31, that is,a group-III nitride whose in-plane lattice constant is smaller than thatof the group-III nitride of the second composition layer 32. Thetermination layer 4 is formed with a thickness greater than thethickness of the first composition layer 31. Presence of the terminationlayer 4 in such a manner enables the compressive strain introduced inthe composition modulation layer 3 to be suitably maintained even in acase where the first intermediate layer 5 is provided in alater-described manner.

However, in a case where the thickness of the termination layer 4 is toolarge, a lattice constant thereof approaches the bulk state, andtherefore lattice relaxation is caused in the first intermediate layer 5formed thereon. In such a case, the first intermediate layer 5, which isprovided originally in order to function as a strain reinforcing layeras described later, does not exert its functions and effects. It is notpreferable. On the other hand, in a case where the thickness of thetermination layer 4 is too small, it is likely that a tensile stressexists in the termination layer 4, similarly to the first compositionlayer 31 interposed between the two second composition layers 32. It isnot preferable. The above-mentioned requirement that the thickness isequal to or greater than the thickness of the first composition layer31, and the requirement that the thickness is about 3 to 100 nm, arepreferable in terms of not causing such failures.

In a case where the thickness of the termination layer 4 is greater thanthe thickness of the first composition layer 31, warping of theepitaxial substrate 10 is more effectively reduced. This manner ispreferable particularly to obtain the epitaxial substrate 10 having ahigh breakdown voltage and suppressed warping.

(First Intermediate Layer)

The first intermediate layer 5 is formed on the termination layer 4 andmade of a group-III nitride having a composition of Al_(y)Ga_(1-y)N(0≦y≦1) whose in-plane lattice constant under the strain-free state isgreater than that of the group-III nitride of the termination layer 4.The first intermediate layer 5 is formed so as to be in the coherentstate relative to the termination layer 4 and formed on the compositionmodulation layer 3 containing the strain that becomes larger in an upperportion. Accordingly, the first intermediate layer 5 itself contains alarge compressive strain. As a result, in the buffer layer 8, the firstintermediate layer 5 functions as a strain reinforcing layer forenhancing the compressive strain introduced into the compositionmodulation layer 3. Providing such a first intermediate layer 5 can moreeffectively cancel the tensile stress in the epitaxial substrate 10.

When the thickness of the first intermediate layer 5 becomes too large,the amount of warping in the epitaxial substrate 10 increases. This isbecause, as the crystal grows, an accumulation of the strain energyreaches a limit so that the compressive strain is weakened and itbecomes difficult for the lattice to grow while kept in the coherentstate, and eventually the critical film thickness is exceeded toconsequently release the strain energy. Such increase in the amount ofwarping is a factor that causes cracking. To be specific, when thethickness of the first intermediate layer 5 exceeds 500 nm, the amountof warping becomes larger than when the first intermediate layer 5 isnot provided. When the thickness of the first intermediate layer 5 isless than 100 nm, the strain reinforcing effect obtained by the firstintermediate layer 5 cannot be sufficiently exerted. Consequently, whenthe thickness of the first intermediate layer 5 is 100 nm or more and300 nm or less, the compressive strain in the buffer layer 8 is suitablyenhanced.

(Second Intermediate Layer)

As a result of forming the composition modulation layer 3, thetermination layer 4, and the first intermediate layer 5 in theabove-described manner, the compressive strain exists in the entire unitstructure 6. Therefore, it is supposed that a large compressive strainsufficient for preventing occurrence of cracking should be obtained bylaminating a plurality of the unit structures 6. However, actually, eventhough one unit structure 6 is formed immediately on another unitstructure 6, a sufficient compressive strain cannot be obtained in theupper unit structure 6, for the following reasons. The group-III nitrideof the first intermediate layer 5 that is the uppermost layer of thelower unit structure 6 has an in-plane lattice constant, under thestrain-free state, greater than that of the group-III nitride of thefirst composition layer 31 that is the lowermost layer of the upper unitstructure 6. Additionally, the first composition layer 31 is formed witha thickness of only about 3 to 20 nm. Accordingly, if the firstcomposition layer 31 is formed directly on the first intermediate layer5, a tensile strain exists in the first composition layer 31, and thus asufficient compressive strain cannot be introduced in the compositionmodulation layer 3.

Therefore, in this embodiment, the second intermediate layer 7 isprovided between the unit structures 6, thereby preventing occurrence ofthe above-described failures involved in the introduction of the tensilestrain, and causing a sufficient compressive strain to exist in eachindividual unit structure 6.

To be specific, on the first intermediate layer 5 that is the uppermostlayer of the unit structure 6, the second intermediate layer 7 made of agroup-III nitride whose in-plane lattice constant under the strain-freestate is smaller than the group-III nitride of the first intermediatelayer 5 is formed. In the second intermediate layer 7 provided in thismanner, a misfit dislocation caused by a difference in the latticeconstant from the first intermediate layer 5 exists in the vicinity ofan interface with the first intermediate layer 5, but a latticerelaxation occurs at least in the vicinity of the surface thereof, toachieve a substantially strain-free state in which no tensile stressacts. Here, being substantially strain-free means that at least aportion other than the vicinity of the interface with the firstintermediate layer 5 immediately below has the substantially samelattice constant as the lattice constant under the bulk state.

In the unit structure 6 formed on such a substantially strain-freesecond intermediate layer 7, no tensile stress acts on the firstcomposition layer 31 that is the lowermost layer of this unit structure6. Therefore, this unit structure 6 is also formed in such a manner thata compressive strain suitably exists therein, similarly to the unitstructure 6 immediately below the second intermediate layer 7.

Preferably, the second intermediate layer 7, similarly to the firstcomposition layer 31, is made of AlN. In this case, by continuouslyforming the second intermediate layer 7 and the first composition layer31, both of them are configured as substantially one layer. This canmore surely prevent a tensile stress from acting on the firstcomposition layer 31.

Here, in order to suppress occurrence of cracking in the epitaxialsubstrate 10, it is necessary that the second intermediate layer 7 isformed with a thickness of 10 nm or more and 150 nm or less. In a casewhere the thickness of the second intermediate layer 7 is less than 10nm, a tensile stress acts on the second intermediate layer 7 similarlyto a case where the first composition layer 31<1> is formed directly onthe first intermediate layer 5, and the composition modulation layer 3is formed under an influence thereof. As a result, a compressive straindoes not suitably exist in the composition modulation layer 3. It is notpreferable. On the other hand, in a case where the thickness of thesecond intermediate layer 7 is more than 150 nm, an influence of adifference in the thermal expansion coefficient between the secondintermediate layer 7 itself and silicon of the base substrate 1 is notnegligible, and a tensile stress caused by the difference in the thermalexpansion coefficient acts on the second intermediate layer 7. It is notpreferable. In either case, cracking occurs in the epitaxial substrate10. Forming the second intermediate layer 7 with a thickness of 10 nm ormore and 150 nm or less enables the second intermediate layer 7 to be ina substantially strain-free state, so that a state where no tensilestress acts on the first composition layer 31<1> located immediatelyabove is created. As a result, the crack-free epitaxial substrate 10 isachieved.

In a case of providing more unit structures 6, the second intermediatelayer 7 is interposed between ones of the unit structures 6 in the samemanner as described above, to thereby achieve a state where acompressive strain suitably exists in all the unit structures 6.

In the epitaxial substrate 10 including the buffer layer 8 configured inthe above-described manner, due to the large compressive strain existingin the buffer layer 8, a state is achieved in which a tensile stresscaused by a difference in the thermal expansion coefficient betweensilicon and the group-III nitride is suitably cancelled. Thereby, in theepitaxial substrate 10, a crack-free state is achieved and the amount ofwarping is suppressed to 100 μm or less.

The above-mentioned requirements that the value of n, which representsthe number of laminations of the first composition layer 31 and thesecond composition layer 32, is about 40 to 100, that the number oflaminations of the unit structure 6 is about 3 to 6, and that therelationship of x(1)≧0.8 and x(n)≦0.2 is satisfied, are preferable interms of providing a sufficient amount of compressive strain in thebuffer layer 8 to thereby cancel the tensile stress caused by thedifference in the thermal expansion coefficient.

That is, in the epitaxial substrate 10 according to this embodiment, thebuffer layer 8 is provided by alternately laminating the unit structure6, in which the termination layer 4 and the first intermediate layer 5serving as a strain reinforcing layer are formed on the compositionmodulation layer 3 serving as a strain introduction layer, and thesubstantially strain-free second intermediate layer 7. This causes alarge compressive strain to exist in the buffer layer 8, to suitablyreduce a tensile stress caused in the epitaxial substrate 10 due to thedifference in the thermal expansion coefficient between silicon and thegroup-Ill nitride. As a result, in the epitaxial substrate 10, acrack-free state is achieved and warping is reduced.

Since the buffer layer 8 is formed on the second base layer 2 b in whichan accumulation of strain energy is suppressed as described above, theeffect of canceling the tensile stress is not hindered by any strainenergy accumulated in the second base layer 2 b.

Moreover, repeatedly laminating the unit structure 6 and the secondintermediate layer 7 increases the total film thickness of the epitaxialfilm itself. In general, in a case where a HEMT device is prepared usingthe epitaxial substrate 10, as the total film thickness thereofincreases, the breakdown voltage of the HEMT device becomes higher.Thus, the configuration of the epitaxial substrate 10 according to thisembodiment also contributes to increase of the breakdown voltage.

<Increase of Breakdown Voltage of Epitaxial Substrate>

The epitaxial substrate 10 according to this embodiment is alsocharacterized by high breakdown voltage properties because of providingof the buffer layer 8 (and more specifically the unit structures 6)having the above-described configuration.

For example, in the epitaxial substrate 10 in which the compositionmodulation layer 3 is formed so as to satisfy the relationship ofx(1)÷0.8 and x(n)≦0.2 and in which the total film thickness of theentire epitaxial film except the base substrate 1 is 4.0 μm or less, ahigh breakdown voltage of 600V or more is achieved. For the achievementof such a high breakdown voltage, it is not essential to provide thesecond intermediate layer 7 in the epitaxial substrate 10. However, asdescribed above, forming the second intermediate layer 7 with athickness of 10 nm or more and 150 nm or less enables the crack-freeepitaxial substrate 10 having an increased breakdown voltage to beobtained. In this embodiment, the breakdown voltage means a voltagevalue at which a leakage current of 1 mA/cm² occurs in a case where thevoltage is applied to the epitaxial substrate 10 while being increasedfrom 0V.

In a case where the first intermediate layer 5 is made of a group-IIInitride having a composition of Al_(y)Ga_(1-y)N (0.25≦y≦0.4), theepitaxial substrate 10 having a further higher breakdown voltage of 900Vor higher is achieved. Alternatively, the epitaxial substrate 10 havinga higher breakdown voltage can also be obtained by appropriately settingthe number of repetitions of lamination of the composition modulationlayers 3, the total film thickness of the entire epitaxial film, and thetotal film thickness of the second composition layers 32. For example,an epitaxial substrate in which the total film thickness of the entireepitaxial film is 5 μm and the breakdown voltage is 1000V or higher, andan epitaxial substrate in which the total film thickness of the entireepitaxial film is 7 μm and the breakdown voltage is 1400V or higher, canbe achieved.

Additionally, in a case where the thickness of the termination layer 4is greater than the thickness of the first composition layer 31, anepitaxial substrate in which the breakdown voltage is high and warpingis suitably suppressed is achieved.

As described above, in this embodiment, the buffer layer formed byalternately laminating the unit structure, which includes thecomposition modulation layer and the first intermediate layer, and thesubstantially strain-free second intermediate layer is provided betweenthe base substrate and the function layer, the composition modulationlayer being formed by the first composition layer and the secondcomposition layer being alternately laminated in such a manner that theAl mole fraction in the second composition layer decreases in an upperportion. Accordingly, an epitaxial substrate having an increasedbreakdown voltage can be obtained in which a silicon substrate, which iseasily available in a large diameter at a low cost, is adopted as a basesubstrate thereof.

Moreover, since the second intermediate layer is formed with a thicknessof 10 nm or more and 150 nm or less, a crack-free epitaxial substratehaving a high breakdown voltage in which the amount of warping isreduced to about 60 μm to 70 μm is achieved.

<Modification>

In the epitaxial substrate 10, an interface layer (not shown) may beprovided between the base substrate 1 and the first base layer 2 a. Inone preferable example, the interface layer has a thickness of aboutseveral inn and is made of amorphous SiAl_(u)O_(v)N_(w).

In a case where an interface layer is provided between the basesubstrate 1 and the first base layer 2 a, a lattice misfit between thebase substrate 1 and the second base layer 2 b, and the like, is moreeffectively relieved, and the crystal quality of each layer formedthereon is further improved. That is, in a case where an interface layeris provided, an AlN layer that is the first base layer 2 a is formedsuch that the AlN layer has a concavo-convex shape similar to a casewhere no interface layer is provided and such that the amount of crystalgrain boundaries existing therein is reduced as compared with the casewhere no interface layer is provided. Particularly, the first base layer2 a having improvement in the half width value of the (0002) X-rayrocking curve is obtained. This is because, in a case where the firstbase layer 2 a is formed on the interface layer, nucleus formation ofAlN, which will make the first base layer 2 a, is less likely toprogress than in a case where the first base layer 2 a is formeddirectly on the base substrate 1, and consequently the growth in thehorizontal direction is promoted as compared with when no interfacelayer is provided. The film thickness of the interface layer is to anextent not exceeding 5 nm. In a case where such an interface layer isprovided, the first base layer 2 a is formed such that the half width ofthe (0002) X-ray rocking curve can be in a range of 0.5 degrees or moreand 0.8 degrees or less. In this case, the function layer 9 can beformed with a more excellent crystal quality in which the half width ofthe (0002) X-ray rocking curve is 800 sec or less and the screwdislocation density is 1×10⁹/cm² or less.

The formation of the interface layer is achieved by, after the siliconwafer reaches the first base layer formation temperature and before thefirst base layer 2 a is formed, introducing only an TMA bubbling gasinto the reactor to expose the wafer to an

TMA bubbling gas atmosphere.

Furthermore, in the formation of the first base layer 2 a, at least oneof Si atoms and O atoms may diffuse and form a solid solution in thefirst base layer 2 a, or at least one of N atoms and O atoms may diffuseand form a solid solution in the base substrate 1.

Layer configurations (such as a manner of giving the compositionalgrading) of the composition modulation layers 3 included in the bufferlayer 8 should not necessarily be the same, but may be different fromone another.

EXAMPLES

As an example, a plurality of types of epitaxial substrates 10 wereprepared, which were different from one another in terms of the layerconfiguration of the butler layer 8. Table 1 shows a basic configurationof the epitaxial substrates 10 according to the example, and morespecifically, materials for forming the respective layers and the filmthicknesses of the respective layers.

TABLE 1

As shown in Table 1, in this example, the materials and the filmthicknesses of the base substrate 1, the base layer 2 (the first baselayer 2 a and the second base layer 2 b), and the function layer 9 werethe same for all the epitaxial substrates 10. The function layer 9 wasconfigured as two layers of the channel layer and the barrier layer.

Any of the first composition layer 31, the termination layer 4, and thesecond intermediate layer 7 was made of AlN, but the film thicknessesthereof was varied depending on specimens. In Table 1, the filmthickness of the first composition layer 31 is represented as thevariable A (nm), the film thickness of the termination layer 4 isrepresented as the variable C (nm), and the film thickness of the secondintermediate layer 7 is represented as the variable E (nm). In the samemanner, the film thickness of the second composition layer 32 isrepresented as the variable B (nm), and the film thickness of the firstintermediate layer 5 is represented as the variable D (nm). n representsthe number of each of the first composition layer 31 and the secondcomposition layer 32. K represents the number of repetitions of the unitstructures 6.

Example 1

In this example, the values of B, D, E, n, and K, and the compositionalgrading, were variously changed. Thereby, 23 types of epitaxialsubstrates 10 (specimens No. 1 to No. 19, and No. 28 to No. 31) in totalwere prepared. In any specimen, A=C=5 nm was satisfied.

As a comparative example, eight types of epitaxial substrates 10(specimens No. 20 to No. 27) were prepared in each of which only onecomposition modulation layer 3 having the constant Al mole fractionthrough all the second composition layers 32 is provided and neither ofthe first intermediate layer 5 nor the second intermediate layer 7 isprovided. In the comparative example, the same preparation conditions asthose of the example were adopted, except for the second compositionlayer 32.

Table 2 shows, with respect to each specimen, the values of A(C), B, D,E, n, and K, the value of the Al mole fraction x(i) in the i-th one ofthe second composition layers 32 as counted from the base substrate 1side, the value of the Al mole fraction y in the first intermediatelayer 5, the total thickness of the composition modulation layers 3, andthe total thickness of the epitaxial film.

TABLE 2 Total Thickness Thickness of Total of Unit Composition ThicknessSpecimen A B D E Structure Modulation of Epitaxial No. (nm) (nm) (nm)(nm) n K x(i) y (nm) Layer (nm) Film (nm) Example 1 5 15 120 25 30 3 1 −(i/n) 0 725 2225 3090 2 5 15 180 25 30 3 0 785 2405 3275 3 5 15 300 2520 3 0 705 2165 3035 4 5 15 120 25 20 4 0 525 2175 3045 5 5 15 300 25 144 0 585 2415 3285 6 5 15 120 15 15 5 0 425 2185 3055 7 5 15 120 15 12 60 365 2265 3135 8 5 15 180 60 18 4 0 545 2360 3230 9 5 15 180 140 16 4 0505 2440 3310 10 5 15 120 25 20 4 0.25 525 2175 3045 11 5 15 120 25 20 40.4 525 2175 3045 12 5 15 300 25 14 4 0.25 585 2415 3285 13 5 15 300 2514 4 0.4 585 2415 3285 14 5 15 120 25 20 4 1 − 0.06 × i (i = 1 to 10) 0525 2175 3045 0.8 − 0.04 × i (i = 11 to 20) 15 5 15 120 25 20 4 1 − 0.04× i (i = 1 to 10) 0 525 2175 3045 1.2 − 0.06 × i (i = 11 to 20) 16 5 15120 25 20 4 1 − 0.08 × i (i = 1 to 5) 0 525 2175 3045 0.7 − 0.02 × i (i= 6 to 15) 1.6 − 0.08 × i (i = 16 to 20) 17 5 15 120 25 20 4 1 − 0.02 ×i (i = 1 to 5) 0 525 2175 3045 1.3 − 0.08 × i (i = 6 to 15) 0.4 − 0.02 ×i (i = 16 to 20) 18 5 15 120 25 20 4 0.8 (i = 1 to 5) 0 525 2175 30450.6 (i = 6 to 10) 0.4 (i = 11 to 15) 0.2 (i = 16 to 20) 19 5 15 120 2518 4 0.9 (i = 1 to 2) 0 485 2015 2885 0.8 (i = 3 to 4) 0.7 (i = 5 to 6)0.6 (i = 7 to 8) 0.5 (i = 9 to 10) 0.4 (i = 11 to 12) 0.3 (i = 13 to 14)0.2 (i = 15 to 16) 0.1 (i = 17 to 18) Comparative 20 5 15 — — 100 1 0 —2005 2005 2875 Example 21 5 20 — — 80 1 0 — 2005 2005 2875 22 5 25 — —70 1 0 — 2105 2105 2975 23 5 35 — — 50 1 0 — 2005 2005 2875 24 5 15 — —100 1 0.1 — 2005 2005 2875 25 5 15 — — 100 1 0.2 — 2005 2005 2875 26 515 — — 100 1 0.3 — 2005 2005 2875 27 5 15 — — 100 1 0.4 — 2005 2005 2875Example 28 5 15 120 — 20 4 1 − (i/n) 0 525 2100 2970 29 5 15 120 200 204 0 525 2700 3570 30 5 25 120 — 20 3 0 725 2175 3045 31 5 25 120 200 203 0 725 2575 3445

A specific process for preparing each of the epitaxial substrates 10 isas follows.

Firstly, until the formation of the second base layer 2 b, the sameprocedure was performed for any of the specimens. A (111) plane singlecrystal silicon wafer (hereinafter, a silicon wafer) of four incheshaving the p-type conductivity and having a substrate thickness of 525μm was prepared as the base substrate 1. The prepared silicon wafer wassubjected to dilute hydrofluoric acid cleaning using dilute hydrofluoricacid having a composition of hydrofluoric-acid/pure-water=1/10 (volumeratio), and subjected to SPM cleaning using cleaning liquid having acomposition of sulfuric-acid/aqueous-hydrogen-peroxide=1/1 (volumeratio). Thus, a state was created in which an oxide film having athickness of several Å was formed on the wafer surface, which was thenset in a reactor of a MOCVD apparatus. Then, a hydrogen/nitrogen mixedatmosphere was created in the reactor, and the pressure in the reactorwas set to be 15 kPa. Heating was performed until the substratetemperature reached 1100° C. that is the first base layer formationtemperature.

When the substrate temperature reached 1100° C., a NH₃ gas wasintroduced into the reactor, and the substrate surface was exposed to aNH₃ gas atmosphere for one minute.

Then, a TMA bubbling gas was introduced into the reactor with apredetermined flow ratio, to react NH₃ with TMA, so that the first baselayer 2 a whose surface has a three-dimensional concavo-convex shape wasformed. At this time, the growing speed (film formation speed) of thefirst base layer 2 a was set to be 20 nm/min, and the target averagefilm thickness of the first base layer 2 a was set to be 100 nm.

After the first base layer 2 a was formed, then the substratetemperature was set to be 1100° C. and the pressure in the reactor wasset to be 15 kPa. A TMG bubbling gas was further introduced into thereactor, to react NH₃ with TMA and TMG, so that an Al_(0.1)Ga_(0.9)Nlayer serving as the second base layer 2 b was formed so as to have anaverage film thickness of about 40 nm.

Subsequent to the formation of the second base layer 2 b, the bufferlayer 8 was prepared in accordance with the values of A(C), B, D, E, n,K, x(i), and y shown in Table 2. In the formation of the buffer layer 8,the substrate temperature was set to be 1100° C., and the pressure inthe reactor was set to be 15 kPa. The same material gas as for theformation of the base layer 2 was used.

The following is an outline of specific set values for A, B, D, E, n, K,and y in the example and the comparative example.

A(C): example (5 nm) comparative example (5 nm);

B: example (15 nm, 25 nm), comparative example (15 nm, 20 nm, 25 nm, 35nm);

D: example (120 nm, 180 nm, 300 nm), comparative example (not set);

E: example (not set, 15 nm, 25 nm, 60 nm, 140 nm, 200 nm), comparativeexample (not set);

n: example (12, 14, 15, 16, 18, 20, 30), comparative example (50, 70,80, 100);

K: example (3, 4, 5, 6), comparative example (1); and

y: example (0, 0.25, 0.4), comparative example (not set).

In the specimens according to the example, the compositional gradinggiven to the second composition layer 32, that is, the Al mole fractionx(i) in each second composition layer 32<i> among the second compositionlayers 32<1> to 32<n>, was broadly classified into the following threemanners. FIG. 3 is a diagram illustrating how the Al mole fraction waschanged in main specimens. It is to be noted that any specimen wasformed so as to satisfy the relationship of x(1)≧0.8 and x(n)≦0.2.

Nos. 1 to 13 and Nos. 28 to 31: the Al mole fraction x(i) wasmonotonically decreased at a constant rate;

Nos. 14 to 17: the Al mole fraction x(i) was monotonically decreased butthe rate of change in the course of the decrease was varied; and

Nos. 18 to 19: the Al mole fraction x(i) was changed stepwise.

On the other hand, in the specimens according to the comparativeexample, the value of the Al mole fraction x in the second compositionlayer 32 was fixedly set to be any of 0, 0.1, 0.2, 0.3, and 0.4.

For any of the specimens according to the example and the comparativeexample, after the buffer layer 8 was formed, the channel layer made ofGaN and serving as the function layer 9 was formed with a thickness of700 nm, and then the barrier layer made of Al_(0.2)Ga_(0.8)N was furtherformed with a thickness of 25 nm. In the formation of the function layer9, the substrate temperature was set to be 1100° C., and the pressure inthe reactor was set to be 15 kPa. The same material gas as for theformation of the base layer 2 was used.

Through the above-described process, 31 types of epitaxial substrates 10were obtained in total.

For the obtained epitaxial substrates 10, the presence or absence ofoccurrence of cracking was visually checked. Additionally, the amount ofwarping was measured using a laser displacement gauge. Here, for theepitaxial substrates 10 where cracking occurred, the breakdown voltagewas measured in a region not including the cracking. Results of themeasurements are shown in Table 3.

TABLE 3 Specimen Warping Breakdown No. Occurrence of Cracking (μm)Voltage (V) 1 Not Observed 64 855 2 Not Observed 73 855 3 Not Observed67 630 4 Not Observed 71 780 5 Not Observed 72 600 6 Not Observed 63742.5 7 Not Observed 68 720 8 Not Observed 70 720 9 Not Observed 71 66010 Not Observed 63 900 11 Not Observed 67 972 12 Not Observed 75 1080 13Not Observed 76 1260 14 Not Observed 68 780 15 Not Observed 70 780 16Not Observed 74 780 17 Nol Observed 69 780 18 Not Observed 66 620 19 NotObserved 65 860 Comparative 20 Occurred at 20 mm from Outer Periphery135 120 Example 21 Occurred at 20 mm from Outer Periphery 142 145 22Occurred at 20 mm from Outer Periphery 156 180 23 Occurred at 20 mm fromOuter Periphery 156 160 24 Occurred at 20 mm from Outer Periphery 162350 25 Occurred at 20 mm from Outer Periphery 163 420 26 Occurred at 20mm from Outer Periphery 169 510 27 Occurred at 20 mm from OuterPeriphery 171 590 Example 28 Occurred at 20 mm from Outer Periphery 138700 29 Occurred at 20 mm from Outer Periphery 156 700 30 Occurred at 20mm from Outer Periphery 135 850 31 Occurred at 20 mm from OuterPeriphery 165 850

As for the breakdown voltage, in the specimens according to thecomparative example, even the highest breakdown voltage was below 600V,whereas in all the specimens according to the example, the breakdownvoltage was 600V or higher. These results indicate that forming thecomposition modulation layer 3 so as to give the compositional gradingto the second composition layer 32 can provide the epitaxial substrate10 having a high breakdown voltage. Additionally, it was also confirmedthat an epitaxial substrate having a further higher breakdown voltage isobtained when the first intermediate layer is made of a group-IIInitride having a composition of Al_(y)Ga_(1-y)N (0.25≦y≦0.4).

As for occurrence of cracking, in all the specimens according to thecomparative example, occurrence of cracking was observed at 20 mm fromthe outer periphery. Among the specimens according to the example, inthe specimens Nos. 28 to 31, cracking occurred at 20 mm from the outerperiphery. On the other hand, in the specimens Nos. 1 to 19, no crackingwas observed irrespective of the manner of giving the compositionalgrading to the second composition layer 32. That is, among the specimensaccording to the example, in a case where the second intermediate layerhad a thickness of 10 nm or more and 150 nm or less, no crackingoccurred, while in a case where the second intermediate layer had athickness excessively smaller or greater than the above-mentioned range,cracking occurred.

In the specimens where cracking occurred, the amount of warping was atleast 135 μm, which largely exceeds 100 μm, while in the specimensaccording to the example where no cracking occurred, the amount ofwarping was suppressed to about 60 μm to 70 μm.

The above-described results indicate that providing the buffer layer 8,in which the unit structure having the compressive strain containedtherein and the second intermediate layer having a thickness of 10 nm ormore and 150 nm or less are alternately laminated, while the unitstructure includes the composition modulation layer formed by the firstcomposition layer 31 and the second composition layer 32 beingalternately laminated in such a manner that the compositional grading isgiven to the second composition layer 32, the termination layer, and thefirst intermediate layer, is effective in achieving a crack-free stateof the epitaxial substrate 10 and suppression of warping therein.

In the specimens according to the comparative example, the secondcomposition layer has a relatively small thickness, and therefore itwould be guessed that the second composition layer itself grew in acoherent state. Despite this, cracking occurred in the comparativeexample. Accordingly, it is considered that, in a case where, as in thecomparative example, the first composition layer and the secondcomposition layer are merely alternately laminated without anycompositional grading given to the second composition layer, thecompressive strain is introduced into each individual second compositionlayer 32 but the total sum thereof is not sufficient for canceling thetensile stress.

Example 2

In this example, through the same procedure as in the example 1, ninetypes of epitaxial substrates 10 were prepared, which corresponded tothe specimens Nos. 4, 10, 11, and 14 to 19 according to the example 1except that the thickness of the termination layer 4 was greater thanthe thickness of the first composition layer 31. Then, the presence orabsence of occurrence of cracking was evaluated, and the amount ofwarping was measured, and the breakdown voltage was measured. Forfacilitating the comparison, these specimens are denoted by Nos. 4′,10′, 11′, and 14′ to 19′. As for the thickness of the termination layer4, three different levels of thickness, namely, 30 nm, 50 nm, and 70 nm,were adopted. In any of the specimens, A=5 nm, B=15 nm, D=120 nm, E=25nm, and K=4 were satisfied.

Table 4 shows, with respect to each specimen, the values of C and n, thevalue of the Al mole fraction x(i) in the i-th one of the secondcomposition layers 32 as counted from the base substrate 1 side, thevalue of the Al mole fraction y in the first intermediate layer 5, thetotal thickness of the composition modulation layers 3, the totalthickness of the epitaxial film, and results of the measurements of theamount of warping and the breakdown voltage.

TABLE 4 Total Thickness Thickness of Total of Unit Composition ThicknessBreakdown Specimen C Structure Modulation of Epitaxial Warping VoltageNo. (nm) n x(i) y (nm) Layer (nm) Film (nm) (μm) (V)  4′ 30 20 1 − (i/n)0.4 550 2275 3140 61 785 10′ 50 20 0.25 570 2355 3220 53 905 11′ 70 200.4 590 2435 3300 57 965 14′ 30 20 1 − 0.06 × i (i = 1 to 10) 0 550 22753140 58 768 0.8 − 0.04 × i (i = 11 to 20) 15′ 30 20 1 − 0.04 × i (i = 1to 10) 0 550 2275 3140 60 780 1.2 − 0.06 × i (i = 11 to 20) 16′ 30 20 1− 0.08 × i (i = 1 to 5) 0 550 2275 3140 64 775 0.7 − 0.02 × i (i = 6 to15) 1.6 − 0.08 × i (i = 16 to 20) 17′ 30 20 1 − 0.02 × i (i = 1 to 5) 0550 2275 3140 59 785 1.3 − 0.08 × i (i = 6 to 15) 0.4 − 0.02 × i (i = 16to 20) 18′ 30 20 0.8 (i = 1 to 5) 0 550 2275 3140 56 625 0.6 (i = 6 to10) 0.4 (i = 11 to 15) 0.2 (i = 16 to 20) 19′ 30 18 0.9 (i = 1 to 2) 0550 2275 3140 55 870 0.8 (i = 3 to 4) 0.7 (i = 5 to 6) 0.6 (i = 7 to 8)0.5 (i = 9 to 10) 0.4 (i = 11 to 12) 0.3 (i = 13 to 14) 0.2 (i = 15 to16) 0.1 (i = 17 to 18)

As shown in Table 4, in any of the specimens Nos. 4′, 10′, 11′, and 14′to 19′, the amount of warping was further suppressed as compared withthe specimens (Nos. 4, 10, 11, and 14 to 19) according to the example 1prepared under the same conditions except for the thickness of thetermination layer 4. In any of the specimens, no occurrence of crackingwas observed. On the other hand, the value of the breakdown voltage wasalmost similar. These results indicate that the epitaxial substrate 10in which the amount of warping is further reduced is achieved by formingthe termination layer 4 thicker than the first composition layer 31.

1. An epitaxial substrate in which a group of group-III nitride layersare formed on a base substrate made of (111)-oriented single crystalsilicon such that a (0001) crystal plane of said group of group-IIInitride layers is substantially in parallel with a substrate surface ofsaid base substrate, said epitaxial substrate comprising: a buffer layerincluding a plurality of first lamination units each including acomposition modulation layer and a first intermediate layer, saidcomposition modulation layer being formed of a first composition layermade of AlN and a second composition layer made of a group-III nitridehaving a composition of Al_(x)Ga_(1-y)N (0≦x≦1) being alternatelylaminated, said first intermediate layer being made of a group-IIInitride having a composition of Al_(y)Ga_(1-y)N (0≦y≦1); and a crystallayer formed on said buffer layer, wherein each of said compositionmodulation layers is formed so as to satisfy the relationship of:x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n); andx(1)>x(n), where n represents the number of laminations of each of saidfirst composition layer and said second composition layer (n is anatural number equal to or greater than two), and x(i) represents thevalue of x in i-th one of said second composition layers as counted fromsaid base substrate side, in each of said composition modulation layers,each of said second composition layers is formed so as to be in acoherent state relative to said first composition layer, said firstintermediate layer is formed so as to be in a coherent state relative tosaid composition modulation layer.
 2. The epitaxial substrate accordingto claim 1, wherein said buffer layer is formed of said first laminationunit and a second lamination unit being alternately laminated, saidsecond lamination unit is a substantially strain-free intermediate layermade of AlN and formed with a thickness of 10 nm or more and 150 nm orless.
 3. The epitaxial substrate according to claim 1, wherein saidfirst intermediate layer is made of a group-III nitride having acomposition of Al_(y)Ga_(1-y)N (0.25≦y≦0.4).
 4. The epitaxial substrateaccording to claim 1, wherein a termination layer having the samecomposition as that of said first composition layer is provided in anuppermost portion of said composition modulation layer.
 5. The epitaxialsubstrate according to claim 4, wherein the thickness of saidtermination layer is greater than the thickness of said firstcomposition layer.
 6. The epitaxial substrate according to claim 1,further comprising: a first base layer made of AlN and formed on saidbase substrate; and a second base layer made of Al_(p)Ga_(1-p)N (0≦p≦1)and formed on said first base layer, wherein said first base layer is alayer with many crystal defects configured of at least one kind from acolumnar or granular crystal or domain, an interface between said firstbase layer and said second base layer defines a three-dimensionalconcavo-convex surface, said buffer layer is formed immediately on saidsecond base layer.
 7. A method for manufacturing an epitaxial substratefor use in a semiconductor device, said epitaxial substrate having agroup of group-III nitride layers formed on a base substrate made of(111)-oriented single crystal silicon such that a (0001) crystal planeof said group of group-III nitride layers is substantially in parallelwith a substrate surface of said base substrate, said method comprising:a buffer layer formation step for forming a buffer layer including aplurality of first lamination units each including a compositionmodulation layer and a first intermediate layer, said buffer layerformation step including a plurality of composition modulation layerformation steps and a plurality of first intermediate layer formationsteps, said composition modulation layer formation step being a step forforming said composition modulation layer by alternately laminating afirst composition layer made of AlN and a second composition layer madeof a group-III nitride having a composition of AlxGa1-xN (0≦x≦1), saidfirst intermediate layer formation step being a step for forming saidfirst intermediate layer made of a group-III nitride having acomposition of Al_(y)Ga_(1-y)N (0≦y≦1); and a crystal layer formationstep for forming a crystal layer above said buffer layer, said crystallayer being made of group-III nitride, wherein in said compositionmodulation layer formation step, said composition modulation layer isformed in such a manner that: the relationship of:x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n); andx(1)>x(n), is satisfied, where n represents the number of laminations ofeach of said first composition layer and said second composition layer(n is a natural number equal to or greater than two), and x(i)represents the value of x in i-th one of said second composition layersas counted from said base substrate side; and each of said secondcomposition layers is in a coherent state relative to said firstcomposition layer, in said first intermediate layer formation step, saidfirst intermediate layer is formed so as to be in a coherent staterelative to said composition modulation layer.
 8. The method formanufacturing the epitaxial substrate according to claim 7, wherein saidbuffer layer formation step is a step for forming said buffer layer inwhich said first lamination unit and a second lamination unit arealternately laminated, by alternately performing a first lamination unitformation step for forming said first lamination unit and a secondlamination unit formation step for forming said second lamination unit,in said second lamination unit formation step, an intermediate layermade of AlN with a thickness of 10 nm or more and 150 nm or less isformed as said second lamination unit.
 9. The method for manufacturingthe epitaxial substrate according to claim 7, wherein in said firstintermediate layer formation step, said first intermediate layer isformed of a group-III nitride having a composition of Al_(y)Ga_(1-y)N(025≦y≦0.4).
 10. The method for manufacturing the epitaxial substrateaccording to claim 7, wherein said first lamination unit formation stepincludes a termination layer formation step for forming a terminationlayer having the same composition as that of said first compositionlayer in an uppermost portion of said composition modulation layer, andsaid first intermediate layer is formed on said termination layer. 11.The method for manufacturing the epitaxial substrate according to claim10, wherein in said termination layer formation step, said terminationlayer is formed thicker than said first composition layer.
 12. Themethod for manufacturing the epitaxial substrate according to claim 7,further comprising: a first base layer formation step for forming afirst base layer on said base substrate, said first base layer beingmade of AlN; and a second base layer formation step for forming a secondbase layer on said first base layer, said second base layer being madeof Al_(p)Ga_(1-p)N (0≦p≦1), wherein in said first base layer formationstep, said first base layer is formed as a layer with many crystaldefects configured of at least one kind from a columnar or granularcrystal or domain, such that a surface thereof is a three-dimensionalconcavo-convex surface, in said buffer layer formation step, said bufferlayer is formed immediately on said second base layer.
 13. The epitaxialsubstrate according to claim 2, wherein said first intermediate layer ismade of a group-III nitride having a composition of Al_(y)Ga_(1-y)N(0.25≦y≦0.4).
 14. The epitaxial substrate according to claim 2, whereina termination layer having the same composition as that of said firstcomposition layer is provided in an uppermost portion of saidcomposition modulation layer.
 15. The epitaxial substrate according toclaim 14, wherein the thickness of said termination layer is greaterthan the thickness of said first composition layer.
 16. The method formanufacturing the epitaxial substrate according to claim 8, wherein insaid first intermediate layer formation step, said first intermediatelayer is formed of a group-III nitride having a composition ofAl_(y)Ga_(1-y)N (0.25≦y≦0.4).
 17. The method for manufacturing theepitaxial substrate according to claim 8, wherein said first laminationunit formation step includes a termination layer formation step forforming a termination layer having the same composition as that of saidfirst composition layer in an uppermost portion of said compositionmodulation layer, and said first intermediate layer is formed on saidtermination layer.
 18. The method for manufacturing the epitaxialsubstrate according to claim 17, wherein in said termination layerformation step, said termination layer is formed thicker than said firstcomposition layer.